This invention relates to a coherent beam transmitter-heterodyne receiver system. Specifically, the invention relates to such a system which is useful for measuring atmospheric winds. A method of sensing, optically, a remote wind is described in a co-pending application Ser. No. 07/230,321, filed Aug. 9, 1988, for METHOD FOR PROCESSING WIND INTENSITY DATA.
When a coherent beam transmitter, such as a laser, is used to illuminate a remote target, the resultant, scattered speckle field generated by the target is randomly perturbed by atmospheric turbulence as the speckle field propagates back to the location of the transmitter-receiver. When a crosswind is present, this scintillation pattern will move with time across the receiver aperture, and consequently, the time delayed statistics of the speckle field at the receiver are dependent upon the crosswind velocity. Thus, speckle-turbulence interaction has the potential for enabling single-ended, remote sensing of the path-averaged vector crosswind in a plane perpendicular to the line of sight of a target. This scintillation pattern detection technique makes global remote wind sensing, as from a satellite, possible.
Conventional apparatus for detecting scintillation patterns for remote wind sensing include double-pulsed, visible or near-infrared sources, and direct detection. Such devices include relatively large and expensive transmitters, are susceptible of interference by smoke, dust and fog, and require complex electronic systems to interpret the received signals. Pulsed source transmitters are inherently less stable than continuous wave transmitters and, additionally, suffer beam jitter problems.
A continuous wave laser transmitter of modest power level (one or two watts) used in conjunction with an optical heterodyne detector can be used to exploit the speckle-turbulence interaction and measure atmospheric winds in a ground based system. The use of a continuous wave transmitter operating at a wavelength of 10.6 micrometers and an optical heterodyne detector has many advantages including the availability of compact, reliable and inexpensive transmitters, better penetration of smoke, dust and fog than would a visible laser, stable output power, low beam pointing jitter, and relatively simple receiver electronics. In addition, with the continuous wave transmitter, options exist for processing the received signals for the crosswinds that do not require a knowledge of the strength of turbulence. A pulsed wave, coherent beam system is usable with a diffuse target, such as aerosols.
It should be emphasized that the system to be described does not operate on the same principles as a doppler lidar remote wind sensing system. The doppler systems use the aerosols in the atmosphere to scatter some of the transmitted energy from a coherent pulsed laser system back to a receiver where the doppler shift is used to measure the wind magnitude along the line of sight. The continuous wave speckle-turbulence system uses a hard target, such as the earth, a building or foliage, as the scattering medium and measures the path averaged vector wind in a plane perpendicular to the line of sight. A pulsed wave system may be operated with an aerosol-like target. The approaches compliment each other and each has certain advantages. A significant advantage of the doppler system is that path resolved wind is easily obtained. There are potential methods, such as crossed beams and multiple detectors, for achieving some path resolution with a speckle-turbulence system, however, it would greatly complicate the system. The advantages of a speckle-turbulence system are the ability to measure the vector wind in a plane perpendicular to the line of sight and very simple equipment and data processing. By using optical heterodyne detection, only a watt or two of optical power is needed. Consequently, laser diodes may be used as the transmitter source, enhancing portability and reliability.
An object of the instant invention is to provide an optical heterodyne apparatus for measuring atmospheric winds.
Another object of the invention is to provide an apparatus which uses a low power laser as a continuous wave light source.
Still another object of the invention is providing an apparatus for isolating a local oscillator beam from a transmitted beam by frequency shifting one or both beams in the radio frequency range.
Another object of the invention is to provide an apparatus which uses serially arranged acousto-optic modulators for accomplishing such shifting.
Yet another object of the invention is to provide an apparatus that may be constructed and contained in a relatively compact and low cost transmitter-receiver system.
The apparatus of the present invention includes a light source for producing a coherent beam of light. A beam splitter is provided for splitting the beam of light into a first, transmitted beam segment and a second, local oscillator beam. Means are provided for frequency shifting the frequency of one or both beam segments and for directing the first beam segment to a target. A remote target for scattering the first beam is provided. Means are provided for combining the scattered first beam segment returning from the target and the second beam segment into a combined beam, and detecting the combined beam. A detector is operative to generate a signal indicative of the crosswind along the path of the directed first beam segment. Means for determining the wind speed normal to the path from the generated signal are also provided.
These and other objects and advantages of the invention will become more fully apparent as the detailed description which follows is read in conjunction with the accompanying drawings.